It depends on the number of inputs on the drum module. From looking at the owner's manual, it looks like you can add another tom and another cymbal.

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He should be able to add whatever he wants, really... I know on the TD6, there are tom, snare, cymbal, HH, and kick inputs... but i can change the sound to whichever percussion i want it to be. so if he wants another cymbal, but only has a tom input, go ahead and get another cymbal then just change the song from within the unit.